1. Field of the Invention
The present invention generally relates to wet etching of composite oxide structures and, more particularly to patterning of composite oxide structures formed as dielectric stack reflectors, useful as laser ablation masks, by wet etching.
2. Description of the Prior Art
Recent advancements in the construction of electronic devices have permitted the successful fabrication of multi-layer modules such as multilayer ceramic (MLC) devices on a commercial scale. Such devices have the advantages of enabling the inclusion of a plurality of chips, which may be fabricated by different technologies, and providing a network of complex wiring to interconnect such chips. Much of this network of interconnections is formed by the screening of a pattern of a material onto so-called green sheets of uncured ceramic which are then laminated and sintered. Connections between layers of ceramic is accomplished by screening a conductive paste into holes, called vias, through the green sheets. When sintered, the cured paste forms a reliable connection between the conductive patterns on respective layers of the multi-layer device.
A similarly multi-layered so-called distribution structure is formed on the surface of the module for the purpose of making necessary connections between terminals or pads formed on chips and the network of interconnections formed by the multi-layer structure described above. Connections are also made from layer to layer in these distribution structures by means of via holes. The major difference between the distribution structure and the multi-layer connection structure is imposed by the small scale of the chips and the correspondingly high pitch of pad or terminal spacing thereon. To achieve sufficiently high accuracy together with small feature sizes and economically acceptable throughput, laser ablation is used to remove material from the insulating substrates on which conductive patterns are to be formed in order to create the vias by which interlayer connections are achieved.
It is to be understood that laser ablation has application in many diverse fields, including surgery. Laser ablation using a mask in accordance with the present invention is potentially applicable to any laser ablation process which must be done at relatively high laser power and high accuracy with high throughput and a high degree of repeatability. Since these requirements are exemplified by use in semiconductor manufacturing processes and MLC devices, in particular, the mask in accordance with the invention will be disclosed in such an environment. Nevertheless, the potential use of the invention is not to be considered as limited to the manufacture of particular types of devices or even to the fabrication of laser ablation masks in general, regardless of intended application.
In order to form vias accurately at high density and throughput by laser ablation, it is common to use a mask having apertures which are transparent to the wavelength of the radiation output by the laser. The mask can then be used to produce a similar pattern of apertures on each of a plurality of substrates. However, this is usually conducted as a projection process and a high degree of accuracy in alignment of the mask with the workpiece is required.
A further problem is encountered due to the relatively high laser power levels required for acceptable throughput. Such laser power levels often exceed 1 watt/cm.sup.2 and often by a large amount. While ordinary metal (e.g. chromium) masks have been used in the past at lower power levels, power levels above 1 watt/cm.sup.2 cause separation of the metal from an underlying (e.g. glass or quartz) substrate due to absorption of the laser energy by the metal, even though a high percentage of the light may be reflected. The metal of the mask, itself, may be ablated by the laser. Accordingly, the useful life of a particular laser projection mask formed of metal is very limited at high power levels. This problem may also be aggravated by the inability of the mask substrate to conduct heat and thus act as a heat sink for the mask.
Since these projection masks must be formed at extremely high accuracy, the cost of each mask is high. Accordingly, a short useful lifetime of a mask causes the unit cost of the electronic devices fabricated by a process including laser ablation to be greatly increased. The alternative at the present state of the art is to reduce power levels and throughput of expensive machinery which also tends to raise the unit cost of devices produced.
Due to the inability of metal masks to withstand the laser ablation process at desired laser power flux levels, masks composed of alternating dielectric films of silicon oxide and tantalum oxide of closely controlled thickness and differing refractive indices have been proposed and used in some applications. If the thicknesses of the layers are closely controlled with respect to the wavelength of the laser radiation and the respective refractive indices of the materials, a destructive interference pattern can be established to reflect a majority of the light incident on each dielectric layer pair. Desirable thicknesses and materials for these layers are on the order of 500 Angstroms for silicon oxide and 400 Angstroms for tantalum oxide. Even though a significant fraction of the incident light may be passed by each dielectric layer pair, the transmitted radiation flux can be reduced to any arbitrary desired degree by increasing the number of dielectric layer pairs which are stacked together to form the mask.
The materials of such dielectric layer pairs, however, are very difficult to pattern in order to form a mask. Tantalum oxide is especially resistant to etchants normally employed to pattern materials such as inorganic compounds, ceramics, metallic oxides, etc. For this reason, ion milling methods have been used despite the fact that ion milling is an extremely costly process. Furthermore, ion milling is an inherently "dirty" process which produces a relatively high density of opaque defects in the masks produced. While simple, low feature density masks have been produced with acceptable yields, the acceptability of the yields was due to the small number of good cells required in the mask rather than the proportion of defective cells which were produced. In contrast, high feature density masks for producing vias in distribution layers of multilayer structures have not heretofore been fabricated by ion milling at acceptable yields.
For example, the yield that can be projected, based on present ion milling defect density, would be about 4% in a so-called quarter field mask of 25 cells (a 5.times.5 array of cells). For a full field mask of 100 cells, the projected yield would be substantially lower. Accordingly, it is seen that layered dielectric masks have not yet provided a solution to the trade-off between mask cost and machine throughput in semiconductor manufacturing processes involving laser ablation of material.
Dielectric layer pairs of silicon oxide and hafnium oxide have also been previously used in dielectric stack reflectors. Hafnium oxide presented a very similar problem with regard to resistance to most etchants. While hafnium oxide can be etched with hydrogen fluoride or buffered hydrogen fluoride, these etchants attack the boundary between the surface to be etched and the resist. This action causes undercutting of the resist pattern and the hafnium oxide material itself, as well as the silicon oxide layers. Therefore, using this material and etchant, fine features in the laser ablation mask and final product could not be obtained.